This chapter briefly mentions a few of Pasteur's laboratory observations that led him very early to believe that environmental control might be as essential as control of the specific microorganisms for a good management of fermentations and of microbial diseases. There is evidence indeed that Pasteur was at first interested in the potentialities of control by manipulation of the physicochemical environment—what he called the “terrain.”. Pasteur's attitude regarding the importance of physiological well-being in resistance to infection had developed during his studies with silkworms. Pasteur did not hesitate to extend these views to the most important human diseases. This point of view naturally led Pasteur to conclude that resistance to infection could probably be increased by improving the physiological state of the infected individual. Although circumstances did not allow Pasteur to deal effectively with the physiological determinants of infectious diseases, he carried out at least one spectacular experiment concerning the effect of temperature on susceptibility to infection. Pasteur once formulated another hypothesis which, long regarded as naive, is acquiring new significance from recent studies on the effect of nutrition on infection.